1. Field of the Invention
The present invention relates to an electromotive force computing device for computing electromotive force of a secondary battery and a state of charge estimating device for estimating the state of charge of the secondary battery on the basis of the electromotive force of the secondary battery computed by the electromotive force computing device.
2. Description of the Related Art
An electric vehicle, such as a pure electric vehicle (PEV) or a hybrid electric vehicle (HEV) obtaining vehicular driving force from an electric motor, has a secondary battery installed. The electric motor is driven by electric power stored in the secondary battery. Such an electric vehicle is equipped with regenerative braking, namely, a braking function having the electric motor function as a generator during vehicular braking where the kinetic energy of the vehicle is converted into electrical energy. The converted electrical energy is stored into the secondary battery and recycled, such as when performing acceleration.
Since battery performance deteriorates when the secondary battery is over-discharged or over-charged, it is necessary to adjust the charging or discharging operation by recognizing the state of charge (SOC) of the secondary battery. In particular, in a hybrid electric vehicle of the type where a generator is driven by a vehicle-mounted heat engine to generate electric power and the electric power charges the secondary battery, the state of charge may be controlled to an approximate intermediate area (50%-60%) between a fully charged state (100%) and a completely discharged state (0%) so that the secondary battery can receive the regenerative power and so that power can be immediately supplied to a motor if so demanded. In this case, it is preferable to more accurately detect the SOC of the secondary battery.
Japanese Patent laid-Open Publication No. 2003-197272 discloses a method for measuring a charging/discharging current for a secondary battery, multiplying the current value (having a negative symbol for charging and a positive symbol for discharging) with a predetermined charging efficiency η, calculating an integrated capacity by integrating the multiplied value over a time period, and estimating the SOC on the basis of the integrated capacity.
Japanese Patent Laid-Open Publication No. 2003-197272 improves the accuracy of the estimated SOC by correcting the charging efficiency η in accordance with an electromotive force Ve of the secondary battery. However, to improve the accuracy of the SOC by correcting the charging efficiency η in this manner, it becomes important to precisely calculate the electromotive force Ve, which is to be a correction parameter for the charging efficiency η.
The electromotive force Ve is, for example, obtained in the following manner. Namely, multiple data pairs of terminal voltage V and charging/discharging current I of the secondary battery during a predetermined period (for example, 60 seconds) are stored, a primary approximation line (voltage V-current I approximation line) is obtained from regression analysis, and a V intercept of the V-I approximation line is obtained as a no-load voltage V0. Furthermore, an integrated capacity Q is calculated by integrating current I over a predetermined period, a polarized voltage Vp of the battery is obtained on the basis of a change amount ΔQ of the integrated capacity Q during the predetermined period and a battery temperature T, and the electromotive force Ve is obtained by subtracting the polarized voltage Vp from the no-load voltage V0 (refer to Japanese Patent Laid-Open Publication No. 2003-197275).
As described hereinabove, the accuracy of the calculated electromotive force Ve can be improved by calculating the electromotive force Ve while taking the polarized voltage Vp into consideration. However, as shown in FIG. 2 of Japanese Patent Laid-Open Publication No. 2003-197275, the polarized voltage Vp varies with a delay of several tens of seconds from the change amount ΔQ of the integrated capacity. This delay also generates a temporal deviation in the relationship between the no-load voltage V0 and the polarized voltage Vp. Thus, unless this temporal deviation is taken into consideration, an error develops in the electromotive force Ve that is obtained by subtracting the polarized voltage Vp from the no-load voltage V0.
A process for correcting the time delay for the polarized voltage Vp in the procedure for calculating the electromotive force Ve is disclosed in Japanese Patent Laid-Open Publication No. 2003-197272 and Japanese Patent Laid-Open Publication No. 2003-197275.